1. Field of the Invention
The present invention relates to a direct current transmission comprising two separate, parallel transmission circuits, at least one of them being bipolar.
2. The Prior Art
Such a transmission may occur, for example, where a first transmission circuit is associated with a second for increasing the transmission capacity but where it is desirable to keep the two circuits separated from each other to avoid confusing their control systems or their main networks. Another possibility may be that the two circuits are positioned adjacent to each other but still at such a distance from each other that it seems reasonable to keep them separated during normal operation.
By a bipolar transmission circuit is meant a transmission circuit comprising two convertor stations with earthed center taps and connected by way of two conductors with positive and negative polarity, respectively. A pole in such a circuit consists of one conductor and the appropriate convertors in the two stations.
In such a circuit it is known to utilize the transmission conductors as reserves for each other by arranging pole reversers for the convertors in at least one pole and reversing switches for parallel connection of the two poles of the circuit by way of a transmission line. This is made possible by the fact that the transmission lines (transmission conductors) normally have a considerably greater overload capacity than the convertors. A prerequisite for being able to carry out such a pole changing is that the reversible convertors are insulated for full voltage on both terminals, whereas it is usual that the terminal which is normally connected to earth is uninsulated or only rather weakly insulated.
Both the reversing switches, which are normally isolating circuit breakers, and the complete insulation are, however, expensive and the present invention aims at achieving a mutual reserve for the various lines in the above-mentioned parallel transmission circuits with the least possible use of switches and insulation.